Scag Freedom Z with Kawasaki FS600 surging

B737capt

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I have a scag freedom Z model SFZ 48-600FS with a kawasaki FS 600V-CS12-R engine that I bought new back in 2012. It has gotten almost impossible to start hot and now is surging especially at idle. It starts great when cold. It almost sounds like it's going to stall but catches and stays running. It runs great wide open and doesn't bog down. It has 67 hours on the unit. There is a youtube video out of my exact same symptom where a welch plug was missing but mine are all there. The air filter is clean and I just replaced both coils with oem coils, new fuel filter, new spark plugs gapped correctly, adjusted the valve lash and I finally drained all the fuel and replaced with fresh 89 octane which I have always used due to my Stihl tools needing 89. I took the fuel bowl off and was clean. I haven't touched the carb yet but wondered if anyone has any other ideas before I dig deeper. I can't imagine a safety switch somewhere causing this type of malfunction as that would be either on or off I assume. Any ideas would be helpful.
Thanks, Bill

Another thought is it possible that the fuel emissions canister could cause this?
 

mechanic mark

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B737capt

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https://www.scag.com/OPManuals/FreedomZ/2013SFZOPMAN/SFZ48_Book_03322.pdf

remove blower housing, top engine cover, & clean with shop vac, cheap paint brushes etc. remove dirt, grass, etc. from all areas, including engine cooling fins. this is why i suggest using a shop vac to loosen crud up vacuum it up, do not use water. clean underside of deck & check blades for sharpening, let us know if this helps.

do not know if this is your manual above:https://www.scag.com/manuals.html

Thanks, Mark for the response. Back when I pulled the cover to change the coils I used Johnson & Johnson scrubbing bubbles to clean the motor. Been using that stuff for years to clean motorcycle and car engines. I pull the cover routinely to clear debris out. The motor looks like the day it was bought with only 67 hours on it. I keep the blades rotated. I pulled the carb off today and cleaned it and used a rebuild kit to put it back together. Fired it up and same symptoms. Wonder if the fuel pump can cause this as that's about the only thing left to change. Does anyone know if the fuel vapor canister can cause these symptoms?
 

bertsmobile1

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With a governed engine surging is always due to an air:fuel ratio that is too low to maintain the governed speed.

This will either be because not enough fuel is getting to the carb venturi or air is entering after the carb venturi.
You check for the latter by saturating the manifold & head gasket with WD 40 , or similar from a trigger pack.
If the engine blows smoke or changes speed then there is an air leak.

Otherwise you are looking at a fuel restriction
you double check this by closing the choke a little and see if the surging smooths out.

Or you can manually hold the throttle valve open and listen to the engine
Eventually it will start to exhibit lean running.
 

B737capt

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Update on my surging issue. I bought the carb rebuild kit, a fuel pump, and 2 intake manifold gaskets. When removing the pilot jet, I first checked how far out from the seat it was and was 2 turns. After putting everything back together and setting the pilot jet back at 2 turns out, the engine still surged and hard to start hot. I started to play with the pilot jet setting and after setting it 4 turns out, the motor now runs perfectly with no surging. It needed more fuel at the low rpm range. I wonder what could have caused this sudden change in the pilot jet fuel requirements? I thoroughly cleaned the carb out, blew compressed air through the passages and removed all parts that could be removed. I found no dirt anywhere.
 
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